in the wings

Also,waiting in the wings. Nearby in the background, available on short notice. For example, Some police were in the wings in case of trouble at the rally, or There are at least a dozen young managers waiting in the wings for Harold to retire. This expression alludes to the theater, where a player waits in the wings or backstage area, unseen by the audience, for his or her turn to come on stage.
[ Second half of 1800s
]